Law of Demeter

The Law of Demeter (LoD) or principle of least knowledge is a design guideline for developing software, particularly object-oriented programs. In its general form, the LoD is a specific case of loose coupling. The guideline was proposed by Ian Holland at Northeastern University towards the end of 1987, and can be succinctly summarized in each of the following ways:[1]

Each unit should have only limited knowledge about other units: only units "closely" related to the current unit.

Each unit should only talk to its friends; don't talk to strangers.

Only talk to your immediate friends.

The fundamental notion is that a given object should assume as little as possible about the structure or properties of anything else (including its subcomponents), in accordance with the principle of "information hiding". It may be viewed as a corollary to the principle of least privilege, which dictates that a module possess only the information and resources necessary for its legitimate purpose.

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When applied to object-oriented programs, the Law of Demeter can be more precisely called the “Law of Demeter for Functions/Methods” (LoD-F). In this case, an object A can request a service (call a method) of an object instance B, but object A should not "reach through" object B to access yet another object, C, to request its services. Doing so would mean that object A implicitly requires greater knowledge of object B's internal structure.
Instead, B's interface should be modified if necessary so it can directly serve object A's request, propagating it to any relevant subcomponents. Alternatively, A might have a direct reference to object C and make the request directly to that. If the law is followed, only object B knows its own internal structure.

More formally, the Law of Demeter for functions requires that a method m of an object O may only invoke the methods of the following kinds of objects:[2]

O itself

m's parameters

Any objects created/instantiated within m

O's direct component objects

A global variable, accessible by O, in the scope of m

In particular, an object should avoid invoking methods of a member object returned by another method. For many modern object oriented languages that use a dot as field identifier, the law can be stated simply as "use only one dot". That is, the code a.b.Method() breaks the law where a.Method() does not. As an analogy, when one wants a dog to walk, one does not command the dog's legs to walk directly; instead one commands the dog which then commands its own legs.

The advantage of following the Law of Demeter is that the resulting software tends to be more maintainable and adaptable. Since objects are less dependent on the internal structure of other objects, object containers can be changed without reworking their callers.

Basili et al.[3] published experimental results in 1996 suggesting that a lower Response For a Class (RFC, the number of methods potentially invoked in response to calling a method of that class) can reduce the probability of software bugs. Following the Law of Demeter can result in a lower RFC. However, the results also suggest that an increase in Weighted Methods per Class[4] (WMC, the number of methods defined in each class) can increase the probability of software bugs. Following the Law of Demeter can also result in a higher WMC; see Disadvantages.

A multilayered architecture can be considered to be a systematic mechanism for implementing the Law of Demeter in a software system.
In a layered architecture, code within each layer can only make calls to code within the layer and code within the next layer down.
"Layer skipping" would violate the layered architecture.

Although the LoD increases the adaptiveness of a software system, it may (but not necessarily will) result in having to write many wrapper methods to propagate calls to components; in some cases, this can add noticeable time and space overhead.[3][5][6]

At the method level, the LoD leads to narrow interfaces, giving access to only as much information as it needs to do its job, as each method needs to know about a small set of methods of closely related objects.[7] On the other hand, at the class level, the LoD leads to wide (i.e. enlarged) interfaces, because the LoD requires introducing many auxiliary methods instead of digging directly into the object structures [5][6]. One solution to the problem of enlarged class interfaces is the aspect-oriented approach[8], where the behavior of the method is specified as an aspect at a high level of abstraction. This is done by having an adaptive method that encapsulates the behaviour of an operation into a place, with which the scattering problem is solved. It also abstracts over the class structure that results in avoiding the tangling problem. The wide interfaces are managed through a language that specifies implementations. Both the traversal strategy and the adaptive visitor use only a minimal set of classes that participate in the operation, and the information about the connections between these classes is abstracted out.

Since the LoD exemplifies a specific type of coupling, and does not specify a method of addressing this type of coupling, it is more suited as a metric for code smell as opposed to a methodology for building loosely coupled systems.

^ abAppleton, Brad. "Introducing Demeter and its Laws". Retrieved 6 July 2013. A side-effect of this is that if you conform to LoD, while it may quite increase the maintainability and "adaptiveness" of your software system, you also end up having to write lots of little wrapper methods to propagate methods calls to its components (which can add noticeable time and space overhead).

^ ab"Tell, Don't Ask". The Pragmatic Programmers, LLC. Retrieved 6 July 2013. The disadvantage, of course, is that you end up writing many small wrapper methods that do very little but delegate container traversal and such. The cost tradeoff is between that inefficiency and higher class coupling.